Shulou(Shulou.com)11/24 Report--

In 2010, you must take cash with you when you go out

In 2020, you must wear a mask when you go out.

In 2023, you must bring your cell phone when you go out.

No matter working or decompressing, eating or sleeping, a person or a bunch of people, mobile phones are with us all the time. From inseparable partners to powerful bombs, are mobile phones absolutely safe? Friends who are in a hurry to touch the fish can quickly row below to find the answer.

How powerful is the battery? With the increasing demand for the battery life of mobile phones, the capacity of mobile phone batteries is getting larger and larger. Lithium-ion batteries (Li-ion Batteries, hereinafter referred to as LiBs) are used in mobile phones. LiBs plays a very important role from small to headphones, to electric vehicles, and then to energy storage power stations.

At present, the positive electrode of lithium-ion battery of commercial mobile phone mainly uses lithium cobalt, because its volume energy density is the highest, so it can store more energy in very small mobile phone, while the negative electrode generally uses layered graphite. At present, the average output voltage of most mobile phone batteries is about 3.7V, and the capacity is generally 4000mAh, so the energy of a fully charged mobile phone is about 14.8Wh, which is equivalent to 53kJ, that is, the energy of two grenades!

Although the probability of a grenade explosion is very small, no one wants to make the news in this way, right? Therefore, before evaluating the performance of a battery, we must first evaluate its safety, so that consumers can rest assured that the mo uses yu.

Hexagonal warriors evolve into security-centered "pentagonal core warriors" are electrolytes and diaphragms safe? Whether it is a cell, a module or a large energy storage module, it is actually composed of battery units in series and parallel. The cause of thermal runaway is closely related to the basic structure and working principle of the battery:

The composition of lithium-ion battery system [1] the interior of LiBs is generally composed of positive current collector and cathode material, electrolyte, diaphragm, negative electrode material and negative current collector, gasket and shell, and the whole system should also include external circuit, which is simply assumed to be composed of power supply / appliance, switch and connected by wire.

Among them, the electrolyte of LiBs is generally flammable and toxic organic solution, which will decompose at high temperature, so the battery needs to be strictly sealed. If we accidentally disassemble the cell phone battery, we can only see the ear of the positive and negative current collector exposed on the outside, which is used to conduct electrons from the outer circuit (while the cylindrical battery welds the ear directly into the shell, so it can't be seen). In addition, sealing can also ensure that when the battery is under pressure, the electrolyte will not leak and it will be everywhere.

Several common types of lithium-ion battery therefore, the battery material limits its working temperature can not be too high, not only because the electrolyte is easy to decompose, commercial lithium cobalt positive electrode at high temperature, the oxygen inside the lattice will also be removed, not only damage the electrode structure, but also produce oxygen, and the direct reaction with the electrolyte leads to thermal runaway, the general working temperature of commercial lithium-ion battery should not exceed 55 ℃.

So, when the phone is very hot in summer, stop playing and let it stay cool wherever it is.

Not only in the manufacturing process, the battery should always maintain a sealed state when working, in the occurrence of collision, extrusion and other extreme conditions, to ensure that the tightness of the battery will not be affected, of course, the battery will carry out a series of safety tests before leaving the factory. But if the phone is seriously bent and deformed, and you really don't want to change the phone, you also have to change the battery, otherwise you are really holding two grenades in your pocket, and you say it is exciting or not.

In addition to the electrolyte, the diaphragm of LiBs is also one of the main causes of thermal runaway. The electrolyte and diaphragm of LiBs are almost electronically insulated, preventing electrons from the positive electrode from running directly to the negative electrode, resulting in direct discharge of the internal circuit. If the diaphragm is damaged, such as the diaphragm is pierced, the positive and negative electrodes of the battery will come into contact directly under pressure, resulting in a short circuit inside the battery, and then the battery will instantly discharge at a large current, causing the battery to heat out of control.

Therefore, in the manufacture and use of the battery, it must be ensured that the electron can only go through the outer circuit and the ion can only go through the inner circuit, and the two move at the same time to form a closed loop.

Can the battery be recharged indefinitely? When the LiBs is charged, the cathode material loses electrons, the lithium ion takes off, and the positive potential increases. Driven by the electric field, lithium ion diffuses to the negative electrode in the electrolyte, along with the negative electrode to get electrons, lithium ion is embedded in the negative electrode material, the negative electrode potential decreases, and the electric energy is converted into chemical energy and stored in the battery.

However, the battery can not be charged indefinitely, because the positive potential can not exceed the upper limit of the electrochemical stability window of the electrolyte, and the negative battery can not be lower than the lower limit of the electrochemical stability window of the electrolyte, otherwise the composition of the electrolyte will have redox reaction with the electrode, resulting in the decomposition of the electrolyte. The decomposition of the electrolyte will produce flammable or combustion-supporting gases, which will make the battery expand and deform, and generate heat at the same time, resulting in heat out of control.

Schematic diagram of electrode potential change during charging of lithium-ion battery [2] another reason is that the vacancy of negative electrode to hold lithium ion is limited, for example, the theoretical specific capacity of commercial graphite anode is 372mAh / g, and the actual specific capacity is about 350mAh / g. When the vacancy of graphite negative electrode has been filled by lithium ion, the diffused lithium ion can only accumulate on the surface of graphite. If the charge continues, the graphite potential (the lowest lithium intercalation potential is about 0.1V to lithium) continues to decrease. When it is reduced to 0V to lithium, the lithium ion will directly obtain the electrons of the negative electrode and reduce to form metal lithium (dendrite) on the graphite surface. Irregular lithium deposition will pierce the diaphragm, causing the battery short circuit, and then lead to thermal runaway events.

In addition to overcharging, the graphite negative electrode will also cause lithium dendrite to pierce the diaphragm when the battery is charged at low temperature. At low temperature, the lithium intercalation rate of graphite negative electrode becomes slow. if the kinetics is less than the diffusion rate of lithium ion in electrolyte, the lithium ion enriched on the surface of graphite negative electrode may combine with electrons directly and deposit irregularly on the surface of graphite negative electrode to form lithium dendrite. Especially in the case of fast charging at low temperature, the kinetic diffusion velocity inside the electrode is required to be higher, and the phenomenon of lithium deposition is more likely to occur. Therefore, the use temperature of LiBs is generally more than-20 ℃. And it's-20 ℃, so you might as well put the phone in Gazi's nest to keep warm.

Combined with the previous temperature, the LiBs operating temperature range of-20 to 55 ℃ will be safer, and the dynamic performance of the battery will be better.

When lithium dendrites pierce the diaphragm when the battery is fully charged, or when the battery is in a charged state (SOC > 0), connected to the external circuit, the battery will discharge spontaneously, which occurs in the opposite process of charging. When the negative potential increases, the positive potential decreases, approaching the middle like the water level at both ends of the U-tube (but not flat). The battery converts the chemical energy of the electrode into electrical energy (and thermal energy) to power the external circuit. Battery discharge is a spontaneous process, like a waterfall "water rushing down from a height of 3,000 feet." your phone prompts 10% before you play.

Of course, when designing batteries, manufacturers have carried out a variety of safety protection and tests to ensure that there will be no problems when we use them normally or when we occasionally overuse them, such as over-charging and stopping charging, automatic shutdown at high and low temperatures, explosion-proof testing, and so on, so that we do not have to be nervous when using mobile phones. However, if we want to put an end to this danger around us, we should pay attention to the following points:

Use batteries produced by regular manufacturers, preferably from the original factory

When the mobile phone or battery is seriously deformed, replace the battery in time

Do not continue to use the battery at high or low temperatures

Batteries can be equipped with better cooling and heating systems

Do not overcharge or discharge

From the root: play less with your mobile phone

Safer batteries? In addition, to get a safer lithium-ion battery, there are many ways to design materials, such as a few chestnuts:

If the stability of lithium cobalt positive electrode is not enough, the positive electrode is changed. Use a more stable and safe cathode-lithium iron phosphate: the structure is more stable, it will decompose at 800 ℃, and the cycle life is longer, up to 10 years under normal use. In addition, lithium iron phosphate does not contain precious metals and is cheaper, so it is suitable for many scenarios that do not require high energy density.

If the organic electrolyte is unstable, change the electrolyte. Use a more stable and safe electrolyte-water electrolyte: replace the electrolyte solvent with water, which itself is non-toxic and non-flammable, and sometimes even flame retardant. However, the reason why the water lithium-ion battery is not included in the commercial system is that the electrochemical window of pure water is relatively narrow (1.23 V), and there are few choices between positive and negative electrodes, resulting in low output voltage and energy density. However, its safety is beyond doubt, and the water system battery is expected to occupy a place for the large-scale scenic integrated storage power station in the future.

If the diaphragm is not strong enough, change the diaphragm? Just remove the diaphragm! Using all-solid-state lithium-ion battery, the liquid electrolyte and diaphragm are replaced by solid electrolyte, replacing combustible materials, so that the battery is intrinsically safe. At present, solid-state lithium-ion batteries are developing rapidly, and the major electric vehicle companies are competing for the layout.

Because the positive electrode, electrolyte and negative electrode are all solid state, the mechanical contact with poor solid-solid interface is a problem to be solved, but solid-state battery can use metal lithium with higher energy density as negative electrode and stable and reliable solid-state electrolyte. It makes the energy density and safety performance of lithium battery improve again, and it is a strong competitor of power battery in the future.

The structure of all-solid-state lithium-ion battery [5] hand-rubbed battery together! All of you who must be watching the future Jieqing and thousands of people are eager to rub the battery by hand, so join the E01 Group of the Institute of Physics of the Chinese Academy of Sciences and create your own battery Tiwatt here.

Finally, there is no absolute security, only absolute security awareness. But the safest thing is to use your cell phone less. I hope you can use your mobile phone correctly, instead of becoming mobile phone playmates like Xiaobian.

So travelers, every day, ask: what are you turning on your cell phone for?

reference

[1] Goodenough J B, Park K-S.The Li-Ion Rechargeable Battery: a Perspective.Journal of the American Chemical Society,2013, 135 (4): 1167-1176.

Goodenough J B, Kim Y.Challenges for Rechargeable Li batteries [J]. Of Materials,2010, 22 (3): 587603.

[3] Doughty D H, Roth E P.A General Discussion of Li Ion Battery safe Electrochemical Society Interface,2012, 21 (2): 37.

Armand M, Tarascon J M.Building better batteries.Nature, 2008, 451 (7179): 652,657.

[5] Janek J, Zeier W G.Challenges in speeding up solid-state battery development [J] .Nature Energy,2023, 8 (3): 230,240.

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: TT

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